Papermind - Papermind Slack App is a collaborative article-editing and document management platform for Slack
Twilio - Brings voice and messaging to your web and mobile applications.
Slenke - Project management & team collaboration
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Trainual + Slack Integration - Capture knowledge or access training right inside Slack
Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.